Resolution: Works As Intended
What I expected to happen was the carpet would burn and the flame would be in the same block.
What actually happened was there was a gap of the rest of the block and there was fire in the block above.
Steps to Reproduce:
1. Place Netherrack and light.
2. Make a 3x3 square of glass panes (any colour)
3. Place carpet on top and wait.
I think this is because the fire renders like it would for another solid block.